1. Field of the Invention
The present invention is broadly concerned with an apparatus and method for the sensing and detection of carbon monoxide in gaseous mixtures. More particularly, it is concerned with such an apparatus and method of high sensitivity, selectivity and response range, whereby the invention provides a means of detecting and quantifying if desired carbon monoxide present in a wide variety of test samples. The invention makes use of the specificity of coordination electrochemistry, and in practice employs an electrical cell having electrolyte with hydrated Cu(II) ions therein; an electrical potential difference of appropriate magnitude is created between the cell electrodes, so that the Cu(II) ions are reduced to Cu(I) ions which react with carbon monoxide in the test sample to form Cu(I)-carbonyl complexes. An electrical parameter indicative of the reduction of Cu(II) ions to Cu(I) ions (e.g., the response current generated between the electrodes where the potential difference therebetween is maintained at a constant magnitude) is detected as a means of determining the presence and concentration of carbon monoxide in the test sample.
2. Description of the Prior Art
In recent years there has been a rapid growth in the development and use of chemical sensors for a wide range of chemical species. One particular area for which a number of devices have been commercially available for several years is for the detection of oxidizable gases such as carbon monoxide.
Carbon monoxide detection is of importance from two principle standpoints. The first is toxicity, which derives from the ability of carbon monoxide to preferentially bind to hemoglobin in place of oxygen, resulting in a legal work-place exposure limit of 50 ppm. The second is the analysis of the exhaust and flue emissions from the combustion of fossil fuels, both from considerations of optimization of engine/boiler performance as well as pollution control.
Apart from purely chemical methods, such as following the progression of a color change along a sample tube, carbon monoxide is usually selectively monitored (as opposed to total combustibles) using either infrared absorption or amperometrically in an electrochemical cell. The latter typically use a high surface area electrode behind a gas permeable membrane, held at a potential greater than that required for carbon monoxide oxidation. This ensures that the current measured is mass transport limited and, therefore, proportional to the carbon monoxide concentration. Since the oxidation of carbon monoxide requires very positive potentials, these sensors frequently also respond to other oxidizable gases such as hydrogen or methane.
The chemical literature contains a wealth of coordination chemistry involving small molecules and, in principle, such reactions could impart high selectivity for a particular species by a chemical sensor. There are a number of commercially available electrochemical sensors which use coordination chemistry to achieve selectivity for the desired analyte. Examples include ion selective electrodes (Koryta, Anal. Chem. Acta, 233, 1, (1990); Koryta et al., Ion Selective Electrodes, 2nd Edition; Freiser, Ion-Selective Electrodes In Analytical Chemistry, Vols. 1 & 2) and a recent amperometric sensor for carbon dioxide (Evans et al., Journal of Electroanalytical Chemistry, 262, 119, (1989); Analytical Chemistry, 61, 577 (1989).
The literature also contains many examples of the interaction of carbon monoxide and Cu(I); also, coordination of olefins with Cu(I) has been reported. For example, the absorption of carbon monoxide by aqueous solutions containing Cu(I) halide solutions has been known for over a century, and in the intervening years an extensive technology for carbon monoxide separation and purification has been developed based on these systems. Almost all of these techniques involve use of Cu(I) in the presence of a variety of ligands such as chloride, ammonia, lactate, etc. Some work has been done in near neutral solution in the absence of other complexing ligands; these studies include the preparation and isolation of Cu.sub.2 (CO).sub.2 SO.sub.4 by Joannis (Comptes Rendes Hebdomadaires des Seances del Academie des Sciences, 125, 9948 (1897)), by reacting copper sulfate solution and finely divided copper in the presence of carbon monoxide, and, more recently, isolation of Cu(I) carbonyl as the perchlorate salt (Ogura, Inorganic Chemistry, 15, 2301 (1976)). Other examples of the synthesis of Cu(I) carbonyl complexes can be found in the review by Bruce (Journal of Organometallic Chemistry, 44, 209 (1972)). The Cu(I) carbonyl species present in aqueous solution is dependent upon experimental conditions. For example, Busch et al. (Inorganic Chemistry, 18, 521, (1979)) concluded from potentiometric experiments that the principle Cu(I) carbonyl species present in hydrochloric acid solution is CuCOCl.sub.2. Souma et al. (Inorganic Chemistry, 15, 969 (1970)) report that in aqueous BF.sub.3, Cu(CO).sub.n.sup.+ is formed, where n=1 to 4, and their studies have also shown that Cu(CO)X can be prepared in acidic solution, where X is Cl-, HSO.sub.4.sup.-, etc. It would appear that a variety of Cu(I) carbonyl species can be generated, and the equilibria between them is dependent upon experimental conditions. This parallels the versatile chemistry of Cu(I) complexes with halides, phosphines, etc., in nonaqueous solvents.
U.K. published application No. 2,094,005 describes an electrochemical gas sensor for the detection of carbon monoxide. In this system, at the anode, carbon monoxide is electrochemically oxidized, whereas at the cathode a reduction occurs, so that the overall cell reaction is CO+1/2O.sub.2 =CO.sub.2. The sensor apparatus makes use of stacked electrodes and an electrolyte supply, with appropriate wick(s) being employed to ensure operative contact between the electrolyte and electrodes.